Counter current mode

The liquid mean residence time, tm, in counter-current mode was significantly lower than in co-current mode. Statistical analysis (Yates method of ANOVA) indicates that along with the liquid flowrate, the direction of liquid travel is the most significant factor in the liquid mean residence time. This variation in the liquid phase mean residence time, suggests an increase in short-circuiting or channeling in the counter-current mode as a result of the gas-liquid interactions. 

An investigation into the applicability of numerical residence time distribution was carried out on a pilot-scale annular bubble column reactor. Validation of the results was determined experimentally with a good degree of correlation. The liquid phase showed to be heavily dependent on the liquid flow, as expected, but also with the direction of travel. Significantly larger man residence times were observed in the cocurrent flow mode, with the counter-current mode exhibiting more chaimeling within the system, which appears to be contributed to by the gas phase. 

Both batch and continuous adsorption processes are used. In a batch process, the adsorbent bed is allowed to become saturated with adsorbed material and is subsequently regenerated in a cyclic manner. In a continuous process, usually the counter-current mode is adopted for adsorption and desorption, either in time form or in simulated mode. Continuous operation offers many advantages with respect to the efficiency of adsorbent utilization. Thus, for... 

Figure 2 XRD pattern of sample produced in the nebuliser reactor, counter-current mode...

Consider Figure 1, which depicts the conventional hemodialysis system in counter-current mode. This dlalyzer set-up maximizes the concentration difference across the membrane and thus ensures maximum solute transfer. However this configuration also maximizes the transmembrane hydrostatic pressure difference and thus, maximizes the water flux. During hemodialysis the hydrostatic pressure of the blood must always be higher than the pressure of the dialysate to ensure sterility in the event of a membrane rupture. One has the following constraints during... 

In the counter-current mode the magnitude of this difference is set by the construction of the dialyzer and the dialysate pressure control and is generally on the order of 50 mm Hg or greater. This minimum pressure will induce an absolute minimum ultrafiltration rate of 350 ml/hr for a typical high flux membrane. Thus, when the patient has lost sufficient water or perhaps when he does not need to lose any water during dialysis, the patient must continuously be given sterile saline to make up for the minimum ultrafiltration loses. 

The heat exchanger is thermodynamically more efficient when its fluids nin in counter-current mode, as illustrated in Figure 20.1, and accordingly this mode of... 

Figure 20.1 Shell and tube heat exchanger in counter-current mode.

Multiple reactor constructions for gas-liquid reactors are available, because of the large number of different application areas. Spray columns, wetted wall columns, packed columns and plate columns are mainly used for absorption processes. The gas concentrations are low in the case of absorption processes, hence a large interfacial contact area between the gas and the liquid is important to enhance the absorption process. These column reactors usually operate in counter-current mode. Counter-current operation is the optimal operating mode, because at the gas outlet where the gaseous component concentration is lowest, the gas is in contact with a fresh absorption solution. The low concentration of the gaseous component can then partly be compensated by the high concentration of the liquid component. 

This special case in Equation 3.28 is valid for any system, regardless of the phase equilibrium properties of the system. The significance of = — 1 is that this represents the point where the vapor flowrate in the CS tends to zero, and can be seen as the switching point from a counter-current mode of operation to a cocurrent operation mode. The only pinch point at these conditions is thus at Xa and profiles will resemble the mixing only process shown in Figure 3.12b. More importantly, though, this shows that any pinch point loci will always move through Xa, irrespective of phase equilibrium. 

When the separation factor is close to one, i.e. the distribution ratios of a pair of analytes are too close to obtain complete separation in a single stage separation, then multistage separations, usually in a counter current mode, must be employed (see Chapters 15 and 16). 

A schematic representation of these two possibilities is shown in Fig. 1.1. The reaction chamber is the inner tube where a fixed bed of catalyst is assumed to be present. The permeate chamber is the annular region between the inner tube and the shell. In the permeate region, the sweep gas flows in co-current mode in the case (a) and in counter-current mode in the case (b). One or more products of the reaction are shown to flow through the membrane from the catalytic-fixed bed toward the permeate region. However, in co-current scheme an inversion of the flux is theoretically possible at some distance from the inlet section since the flux... 

Obviously, this anomaly is not generally allowed in counter-current mode where the end section of the reactor is characterized by low pressures of removable compounds in the reacting region but by a practically pure sweep gas entering into the permeate region. 

Basile A, Tosti S, Capannelli G, VituUi G, lulianelli A, Gallucci F, Drioli E (2006) Co-current and counter-current modes for methanol steam reforming membrane reactor experimental study. Catal Today 118 237-245... 

Using a counter-current mode leads to completely different partial pressure profiles in reaction and permeation sides with respect to the co-current mode (independently on the reaction system considered), as demonstrated in Figure 3.7. 

In the case of perovskites, oxygen ions and electrons are transported in a counter-current mode and no external electrons are needed the oxygen partial pressure difference through the membrane acts as a driving force. 

Practically all of the industrial extraction processes are run in the counter-current mode, i.e., the fresh solvent meets an aqueous solution that has already been through all extraction steps except the last one. In such a counter current battery the fraction of the element of interest that remains in the aqueous phase is... 

Under equilibrium extraction condition the solvent mixer was loaded in a two stage continuous counter current mode as determined by McCabe Thiele plot using O/A phase ratio of 1 1 (Fig. 5). The raffinate contains only 11 ppm of cobalt along with 30.3 g/L Ni. The loaded organic... 

C (Tmax=1.35). This case represents approximately the minimum cooling required for the PFR. Surprisingly, co-current coolant flow is preferable in this case since the temperature rise is lower compared to the counter-current case. Furthermore, stable operation is obtained under all conditions for co-current cooling, whereas this does not hold for the counter-current mode. 

Figure 1 illustrates the THIOPAQ process. In the scrubber, the H2S-containing gas is contacted with a scrubbing liquid in counter-current mode. Absorption of H2S in the scrubber occurs vmder slightly alkaline conditions (pH 8-9) as follows (28) ... 

The steam generators are of a "once-through" type and implement helical tubes. The flows of the primary and secondary systems circulate in a counter-current mode. The secondary fluid circulates upward within the tubes. It flows into the tubes as liquid-water and it reaches the exit as an overheated steam. 

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